[ifcombine] fix mask variable test to match use [PR118344]

There was a cut&pasto in the rr_and_mask's adjustment to match the
combined type: the test on whether there was a mask already was
testing the wrong variable, and then it might crash or otherwise fail
accessing an undefined mask.  This only hit with checking enabled,
and rarely at that.


for  gcc/ChangeLog

	PR tree-optimization/118344
	* gimple-fold.cc (fold_truth_andor_for_ifcombine): Fix typo in
	rr_and_mask's type adjustment test.

for  gcc/testsuite/ChangeLog

	PR tree-optimization/118344
	* gcc.dg/field-merge-19.c: New.